It is well known in the art of ultrasonic measurements that where the ultrasonic pressure wave must travel through a given measuring path medium, air, for example, which may have one or more temperature gradients therealong, that the range measurements of ultrasonic pressure waves traveling from. An ultrasonic transducer to a target and back to the ultrasonic transducer can vary because the speed of sound traveling in the medium is affected by such temperature gradients. The media having two different temperatures are in effect two different medias to thereby cause a refraction of the pressure wave traversing one body of media having a first temperature to a second body of the same media having a different temperature.
According to the present invention, a flow of measuring path media, between the ultrasonic transducer and the target, provides a uniform temperature between the target and the ultrasonic transducer wherein the temperature (or velocity of sound in the medium in the plenum chamber) is measured in a plenum chamber prior to flowing of the medium in the measuring path. When the measuring path medium is air, it is transported through a path such that the volume of air between the source of issuing the air into the path between the transducer and the target is such that the mass tends to eliminate any sharp step rise in temperature of the air. As noted above, the temperature of the air is measured inside of a plenum chamber. One wall of the plenum chamber serves as a mounting surface for the ultrasonic transducer which may be of the Polaroid.TM.type or a barium titanate, piezoelectric crystal, etc. A plurality of air flow apertures is framed around the transducer and the plenum chamber itself is coupled to a supply of ambient air. In this case, cowl or other chamber-forming means has a length which is less than the ring-down distance (which is equal to the CT/2 where C is the speed of sound in the medium, and T is time). The air flow buffers the temperature and, as noted above, thereby avoids quick temperature changes. According to the present invention measurement of the temperature (or velocity of sound in the medium at a given temperature) is done in the plenum chamber prior to the medium (air) exiting the apertures surrounding the ultrasonic transducer. The thermal mass of the supply system buffers changes in temperature. This location of the temperature (or velocity of sound sensor) rather than in the transducer to target path has been found to be most advantageous.